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Publication numberUS2517891 A
Publication typeGrant
Publication dateAug 8, 1950
Filing dateFeb 10, 1945
Priority dateFeb 10, 1945
Publication numberUS 2517891 A, US 2517891A, US-A-2517891, US2517891 A, US2517891A
InventorsPhillip B Laeser
Original AssigneeJournal Company
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and apparatus for satellite broadcasting
US 2517891 A
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Description  (OCR text may contain errors)

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8, 1 v P. B. LAESER 2,517,891

METHOD AND APPARATUS FOR SATELLITE BROADCASTING Filed Feb. 10, 1945 I INVENTOR PH/u/P 5 LAM-see A TO RNEYS Patented Aug. 8, 1950 BROADCASTING;

Phillip B. Laesen wauwatosa, Wis, assignor to Q The Journal Company, Milwaukee, 'Wis.,-' a corporation of Wisconsin I finvention relates to improvements in methodand apparatus forsatellite broadcasting.

While the principles herein disclosed may be used in sojcalled repeater broadcasting stations, itis; apriina'ry object of the invention to provide improved service ior shadowed" zones in the servicearea 'of a broadcasting station and particularly'such a station as broadcasts at high frequency J "More' or less arbitrarily, the primary service afe'aof abroadcasting'station is one in which a 1000 microvolt" signal is or should be received. For high frequency broadcasting, particularly as represented in the FM field, the outer limits of the" service area of the broadcasting station are defined by field intensities within the 50 microvolt contour. Within theservice areas, there may be existing zones known as shadows and in which obstructions, such as hills,clifis, orbuildings may introduceattenuation to the point where adequate 'signals'cannot be received from the transmitter." Such 'a condition presents a serious problem, especially if 'ashadow falls inside the primary service area For examplasuppose the p 'inla fiservice area is cut by a deep valley. Receptionfrom the transmitter maybe perfect on the cliiis at either side, but may be wholly lacking in the thickly populated valley between.

A' practical method of servicing shadow areas by th e' use ofbooster or satellite stations, of which there areth'ree main types.

flhe operation of an independent transmitter on a frequency other than that assigned to the mainfstation requires the use of twochannels. One disadvantage istheloss of frequency identification due to'the useof dual channels. Another disadvantage lies in the fact that the maximum number of channels available for primary service isreduced if one station hasto use two channels to secure' pro'per coverage. v V

lhe operation of a satellite transmitter using the time program on the same channel as that used" by the primary transmitter but without synchronization, causes a zone between the areas served by; the main transmitter and the areas served by the satellite transmitter in which there i's"objectionable interference at'points where the signals approach equal intensity. Such inter'- erenc is particularly noticeable during program pausesfand at times of zero modulation. It may beev'idenced by loss of [clearness in the signals received or by a beat note; of a frequency representing a difference between'the two carriers or the presence of standing waves which cause a re mdfcement or neutralization of, the signal at Application February 10, .s liiili'N -577 i. j 18 Claims. (01. 2501-15) Y thatfp'oint'. Such an 7 desirable unless the booster or satellite transmitter is so'placed that this zone falls in anarea having relatively few potential listeners;

The present'i'nvention' relates to a station of and modulation, and the broadcasting from the satellite transmitter is in complete synchronism with the broadcast from the primary transmitter.

Specifically, it is my purpose to assure the neces-' sary synchronism, while, at the same time, permitting adequate reamplification at the satellite station of the signal received from the primary transmitter.

Other objects or the invention will be ap-- parent to' those skilledin the artupon analysis of thefollowing disclosure of the invention.

l n'the drawing: 9 Figl' '1' is a diagram illustrating the disposition and connection of component pieces of apparatus in an assemblyfor practice of the-inventionn" i Fig. 2 is a plan view diagrammatically illustrat: in'gth'e service areas covered through the'use or theinv'e'ntion; '1

Like parts are designated by the same reference characters throughout the several views. Referring first to Fig. 2, I have illustrated at l a primary broadcasting transmitter which may be assumed, for the purpose of this disclosure, to befbroadcas'tingan FM signal on a carrier fre quencyof 45.5 megacycles. It will, of course, be understood that allvalues stated are'solely by way'of example.

The circle 2 represents the hypothetical limit of'theservice area of the'transmitter l. Howeverflet it be assumed that at 3 there is a hill "or other natural or artificial screen creating at the side of] such hill remote from the transmitter l a zone 4 of shadow extending outwardly from the hill and roughly lying between radii drawn from the transmitter l and passing at either sideof the screen 3. In accordance with the present invention, a satellite station receiving antenna is disposed at 5 and a satellite station broadcasting antennaat 6.- Preferably the antennae chosen and their relation to each other and the screen 3 (which, preferably intervenes between them) are carefully chosen to make the respective antennae of the satellite as highly directive as possible, the receiving antenna 5 being highly directive for reception from the direction of the primary transmitter I and the satellite transmitting antenna 6 being beamed to cover as com arrangement is very unpletely as possible the zone 4 which lies in shadow so far as the primary transmitter l is concerned. The principles involved in the beaming of the receiving and. transmitting antennae are well known.

The present invention is not concerned with the manner in which the result isachieved. So

long as the combined rejection ratio as between beat frequency is created; at the outputof; con verter. 8a, The oscillator: 9= generates this fre-.--- quency" whictnfor purposes of illustration, may

be assumed to bea 1.4 megacycles. Thisfrequency is-pa-sse'd: through the frequency multipliers I and H3! in ea-ch: of which the frequency is. tripled so that-the resulting frequency fed to-the converter 8 from. the oscillator- 9; is nine; times the original, or 39.6 megacycles. I

The-:mixing of: two frequencies-results inrbeat notesv representing the-sum: andthe-difference. of such frequencies. Theyradiofrequency amplifier l z i'sradjrusted-to be'tuned' to: one of the beat notes. For example, the: radio frequency amplifier i2. may be assumed to be tuned to the beatnote; representing th-e difference between thereceived frequency of 45.5megacyclesand the stepped up os-- cillator frequency of.-'3-9.6megacycles. Inrother" words, the radio frequency amplifier 1'2 isztuned topass abandof frequencies which includes the beat note of 5:9 megacycles.

Theireason for changingth'e-received frequency, is because so extremely high a frequency as $5.5 megacyclesrcannotbe" amplified toadvantage. The: beatnotefrequency of 5. 9: megacycles can" readily be amplified to any desired degree with relatively fewer stages: I

Impraetice; at least onestageof amplification. for'the beat frequency is'contai-ned within. the receivingunit A. The output of amplifierlzis then connectedbycoaxial cable 13, preferably undergrou-nd,-to-the satellite transmitting. unit B: whereit may be: further amplifiedin amplifier M, which passesittoanother converter-unit 1:5.

A signal from the oscillator 9 not only passes through frequency multipliers. l0 and- H. but is also- -connectedby the coaxial cable [6; preferably. underground, to frequency multipliers H and I8 in the transmitting.unitB. Therespective units may havetheir own-independent power supplies at I 9- and is, if desired;. to save communicating wire:

' Sincethe frequency multipliers H and 18 are identical-to those shown. at to and H, and since both sets-are fedfro-m'the same source at. 9, the output of frequency multiplier 18 will-beidentical-in frequency: with the output of frequency multiplier H.- Onthe basis of the assumed-figures,- the output from the multiplier l8- will be 39.6 megacycles When. the injection frequency of -390megacycles is mixed in theconverter [5 with theamplifiedfrequency of 5.9 megacycles, itwill again develop beat notes representing the sumand the difference of the respective frequencies Theamplifier'mistuned to accept only copper. lzi-aflie screen directly back of each.

the beat note representing the sum of the amplifled. frequency of 5.9 megacycles and the injected frequency of 39.9 megacycles. This sum is the identical frequency of the signal originally received at 45.5 megacycles. The amplifier 20 preferably includes not merely amplification mechanism but alsoli-miting apparatus which becomes saturated when the signals are excessively strong and thus reduces all signals to the desired level at which the weakest signal can be properly amplified for retransmission. From the amplifier ,Zilthe signalpreferably passes to the power amplifiers 2| and 22 which may be separately powered from a supply source 23. From the output amplifier 22 the signal passes to the satellite transmitting antenna 6 for rebroadcasting Throughout its passage through the apparatus, asherein disclosed, the signal will retain its original modulation. Since a single oscillator at 9 serves to provide the beat frequency in both of the converters G and. 15,. itwill be apparent that the carrier. frequency. aswell as the modulation. will be provided. by the primary transmitter.

Within the limits. of. the. capacityof. the band: pass R.- F. amplifiers; no amount-of wandering of:

the oscillator 9 can. affect thisresult... The. signal broadcast from. the. satellite antenna 6 wilLcorrespond-inall respectswith. the exception of intensity with the signal broadcast fromthe. primary antenna-at i. By amplifyingthe beatirequencywhich in converter. 8,. is. the difference. betweenthe-incoming. frequency andcthe injected frequency, and by-amplifying at 20. the .beatfre quency produced in converter. l5 which. the, sum of the intermediate frequency andlthe. iiijected. frequency, I necessarily. provide. in. converter. i5 an inherent. and. automatic correction for any. deviation. occurring, in converter. L Whatever variationioccurs in converter 8. is.counteracted by an equaland opposite variation in. converter L5 derived. from. the. same. source,. namely, the. oscillator .9. The oscillator 9is'.p1:eferablycrystaleontrolledn I In. practice, thissyst'em has. been usediman. installati'oninwhich. the mutually screened an.- tennae. 511 and .6. arefhighly. directional and have patternsresembling cardioids the combined. re jection ratiobeing. iii-excess. of 1000 times. The: antennae consisted of... horizontal. dipoles with a EGO-watt output wasv usedlto feedthesatelliteantenna 6..

The. results were so excellentrthatit. was difllcult to locate.- the zone of degraded service between the fieldservedby thesatellite. antenna. 5

' andv the field. served bythe. primary antenna l Comparativetestswere-made using: thesame. signal'frequencies, power, andantennasystems, butwith a non synchronous setup. and. it was found that under those conditions. the. degraded. zone. was very apparent...

Using the. satellitesystem as herein. disclosed, audio frequency beat notes. are completely eliminated. Even in the degraded. zones, there:was almost no audible. distortion and-this was prob-- ably attributable. to thedifierence in: timeover. the paths followed by the respective carrier; waves. The degraded zone. was found to be only a fraction of the width. of. the degraded zone noted when. the. transmitters were operated; non-synchronously. in the test above described.

Where. the antennae 5 and 6 of the satellite. station. are properly screened. either by reasonof the. natural topography orby reason. of their mechanical.- electrical. design,.. the. feedback from.

. more or less.

with a portion of the circuit in which the signal frequency is present, forexample R. F. amplifier antenna 6 toantenna willordinarily be neg- 'ligible'andmay not bedete'ctable. However, in so far as feed back exists, it may become evident when'the primary transmitter I is, for any reaan audible beat frequency. For example, this control frequency may lie above 25,000 cycles. It

is estimated that the control frequency with whichxthe carrier is modulated for control pur- :posesneed not, in practice, require more than twenty per cent modulation of the carrier wave, In operative electrical connection 14, I may provide at a discriminator adapted to demodulate the frequencies. with which the carrier wave is modulated.

A-series of filters 26, 21, 28 may be used' to select particular modulation frequencies "and to pass these to the respective rectifiers and/or amplifiers 29, 30 and 3|. Filter 28, for the purpose of the present disclosure, may be a band'- pass filter .which, for example, may pass frequencies in the band between 25,000 and 35,000

, cycles. Any such frequency passed by the filter 28 and rectified and amplified at 3| will operate the relay 32 to close the switch 33 which, for purposes of illustration, is incorporated in the signal transmission line between amplifiers 20 and 2|.

- The filters 26 and 21 may be highly selective.

By way of example, the filter 26 may be arranged to pass a frequency of 27,000 and the filter 21 a frequency of 38,000, both of such frequencies falling within the band passed by filter 28. The amplified frequencies passed by filters 26 and 21 will operate relays at 34 or 35 to control the functioning of the power supply ;23 for thepower amplifiers 2| and 22, whereby to regulate the power with which the satellite station broadcasts. This, again, is by way of example, for any other function of the satellite station could be similarly controlled. In the assumed example, if the super-audible control frequency impressed on the carrier wave at the primary station is 27,000 cycles, the satellite station may be made to broadcast at one power. 'Ifthe impressed control frequency is 33,000 cycles, the satellite station will broadcast at a different power. As long as either control frequency is present in the modulation of the carrier wave "from the primary broadcasting station,the satel- "lite station will operate, but if neither control station will automatically go off the 'air whenever the "primary station does so but, in addition,

iii

any one of a number of satellite stations may, for any special reason selectively .be put off the air by simply interrupting the control frequency in the control room of the primary broadcasting source.

If a number of satellite stations are to be thus controlled, it will, of course, be understood that the filters of the respective satellites will be made responsive to different frequencies or bands of frequencies. It will be obvious in the light, of the foregoing disclosure that any single control frequency may be used to control any single function where a multiplicity of functions as herein disclosed is not required.

I claim:

1. The combination with a broadcasting station having a shadowed area within its service zone, of a rebroadcasting station of the. type adapted to receive a given carrier wave and to. re:- broadcast a correspondingly modulated wave at the same frequency, and comprising a receiving antenna disposed within an unshadowed portion of said service zone and a directional rebroadcasting antenna having as its service zone substantially solely the shadowed portion of the zone first mentioned, the receiving antenna of said rebroadcasting station being connected with a first section comprising a, heterodyne converter unit, means for injecting a heterodyning frequency into said converter unit for generating modulated subtractive beat frequencies therein, means for selectively amplifying said subtractive frequency, and a second section physically spaced bodily from the first section and comprising a second heterodyne converter unit operatively coupled electrically with the first section amplifying means to receive the selectively amplified subtractive beat frequency aforesaid, means'for injecting a heterodyning frequency into'the second converter unit for generating an additive beat frequency therein, and means for selectively amplifying and broadcasting said additive beat frequency, the said additive frequency being identical with the frequency broadcast by said first mentioned broadcasting station and means constituting a single frequency source communicating with the respective means of the separate sections for injecting heterodyning frequencies to the respective converter units, whereby any variation in the operation of said source will be self-compensating as between the respective units.

2. In a broadcasting system of the character described including a primary transmitter designed to service apredetermined area, the combination With a satellite transmitter arranged to service a shadowed portion of the same area, of a receiving antenna arranged to receive a signal from the primary transmitter, a first heterodyne converter and means leading from the receiving antenna for feeding a given carrier frequency thereto, a second heterodynec'onverter, an intervening amplifier operatively connected between the respective converters and adapted to amplify a selected beat note generated'in' the first converter and to deliver the amplified beat note to the second converter, a. second amplifier operatively connected to the output of the second converter and adapted to amplify and deliver a selected beat note generated in the se'condconverter, the amplifiers being tuned to amplify beat notes which are respectively additiv'e'an'd subtractive, a single source of heterodyning frequencies for the respective converters, and means including frequency multiplying means connecte snzgaei combination with a primary broadcaster of modulated carrier frequencies to a predetermined zone, "of means for receiving and rebroadcasting the identical modulated frequencies to a shadowed area in the same zone, said means comprising .first and second heterodyne converters and means for feeding a modulated carrier frequency to the first converter, a single frequency generator provided with means voperatively electrically connecting it with the said converters whereby identical frequencies from said single source are delivered'to the converters, and carrier frequency amplifiers operatively connected to re-- "ceive carrier beat frequencies from the respective converters, said amplifiers being tuned respectively to modulated carrier beat frequencies which are respectively the difference and the 'sum of .the heterodyned frequencies in the re- .spective converters, the first of said amplifiers being operatively connected between the first and 'seond converters and the second of said amplifiers being provided with output connections for the delivery of an amplified modulated frequency identical in modulation with the carrier frequency received by the first of said converters, said receiving and 'rebroadcasting means comprising directional antenna screened from each other and including a rebroadcasting antenna beamed :to the shadowed area of said zone and to substantially no unshadowed portion of said zone, whereby to preclude interference between the signal rebroadcast by said last mentioned antenna and the signal broadcast by said primary broadcaster.

4. In a remotely controlled broadcasting station, the combination with a receiving antenna and broadcasting means including a broadcasting antenna, of an amplifier having its input connected with the receiving antenna and adapted to amplify a modulated carrier wave received by the receiving antenna, a control for said broadcasting means, means for demodulating a modulation frequency of the carrier wave received by the receiving antenna and amplified by said amplifier, and means for delivering the demodulated frequency to said control for the operation thereof and the regulation of said broadcasting means,

said last means comprising a circuit tuned to require the said control frequency to lie within a, super-audible range having no audible beat "-note.

5. The combination set forth in claim 4, in

"which said broadcasting means include a power supply, and said control comprises at least one relay operatively connected to said power supply for the regulation thereof, a filter in operative connection with the relay for passing a particular frequency thereto for the operation of said relay, and a discriminator operatively connected to receive the modulated frequency described in 8 claima t and operativly connectedto sdeliver such frequency to said filter to be passed to said relay forithe regulation of the power supply.

-6. In a satellite broadcasting station including a 'carrier wavegenerator, .a power amplifier and'abroadcas'ting antennajn combination with a receiving antenna for a .modulated carrier wave and tan amplifier in operative connection with said .jcarri'er Wave generator for the modulation of thezcarrier wave'generated in accordance with modulation of the carrier wave received, together with-firstand second controls for said power .amplifier, the first control "being determinative @of the op'erativeness of the connection by which the generated carrier 'wave is modulated, "and the :second control being-determinative .of the'power of said amplifier, relays-in operative connection with the respective "controls, filters connected with the respective relays, one of the filters being :selectivelytuned and the other being a band-pass filter having a range which is super-audible and so high :that no frequency within such rangerhas .aibeat note with the carrier :wav'e whichis audible, said range including the frequency to which the first relay is selectively'tuned, a discriminator operatively connected to receive :and :demodulate :a modulation. frequency 'of the received carrier wave andaoperatively connected *to :deliver :modulation component frequencies :of thereceived'carrier wave to the respective filters, whereby the presence'or absence in the modulation of the received carrier 'wave "of frequencies :passed by the respective filters will operate the respective "con- ,trols through the respective relays.

'7. The combination set forth in claim 4, in which said broadcasting means includes a plu- .ralityof controls, two :of which' respectively control signal cutoif and signal power, "relay :means in operative connection with the respective controls; said means for delivering"demodulated :irequency including filter means tunedto pass-conrtrol frequency modulation components of :said carrier frequency for "selectively operating oneof said controls independently of the other,

8. The method herein disclosed, which "comprises broadcasting a -modulated carrier frequency to a given 'zon'e, receiving and heterofdyning the modulated'c'arrier frequency with :an injected "frequency, amplifying the modulated :beat note resulting from such heterodyning, hetero'dyning the amplified beat note with the identical injected frequency, transmitting from the last mentioned heterodyning operation'the modulate-d beat frequency which is identical to the original modulated frequency, and rebroadcasting the modulated beat frequency to a :shadowed area within said zone "and selectively =ex- (eluding the rebroadcast from substantially all unshadowed portions of said zone, whereby to :avoid interference with the broadcasting first mentioned. 7

-9. The method herein described, which comprises broadcasting a modulated carrier frequency to a predetermined zone, receiving the broadcast modulated carrier frequency within said zone, heterody-ning the received frequency with a given injected-frequencyto generate modulated beat frequencies, an'ip'lifying one of the modulated beat frequencies, heterodyning the amplified modulated beat frequency with :the identical injected frequency thereby developing modulated beat frequenciesyone 'of which is identical with the broadcast carrier frequency originally received, amplifying the modulated beat frequency which is identical with .the carrier ,f-requency originally received, and rebroadcasting and beaming for reception within the same zone the amplified modulated beat frequency last mentioned on the identical frequency of the received signal and in full synchronism therewith, while excluding the beamed rebroadcasting from substantially all portions of said zone except such as are shadowed to prevent access of the original broadcasting thereto.

10. The method set forth in claim 9 including the step of generating a frequency, multiplying the generated frequency and delivering the multiplied frequency from the single source for heterodyning the received frequency and the first beat frequency as set forth in claim 9.

11. The method set forth in claim 8, including the further steps of impressing a, control modulatio'n on the received carrier frequency, demodulating at least a control component of said modulation and controlling the transmission of the output beat frequency in accordance with said control component. 1

12. The method set forth in claim 9, in further combination with the steps of demodulating a modulating component of one of the modulated frequencies mentioned in claim 9, and regulating the rebroadcast aforesaid subject to the control of such modulated component.

13. A method of controlling a satellite broadcasting station from a primary broadcasting station, which method comprises broadcasting a carrier wave from the primary broadcasting station to a predetermined zone, modulating the carrier wave with a super-audible frequency as well as with audible frequencies, receiving and amplifying the modulated carrier wave at the satellite station, demodulating at the satellite station the super-audible frequency modulation of the carrier wave and broadcasting at the satellite station to a portion of the same zone a carrier wave with at least the audible frequency modulation of the original carrier wave, and controlling the broadcasting from the satellite station by said demodulated super-audible frequency.

14. A method of controlling a satellite broadcasting station from a primary broadcasting station, which method comprises the broadcasting of a carrier wave from the primary broadcasting station, modulating said carrier wave with a super-audible frequency as well as with audible frequencies, receiving, amplifying and rebroadcasting the modulated carrier wave at the satellite station, demodulating at the satellite station a component of the modulation of the "carrier wave including the super-audible frequency impressed thereon, amplifying said super-audible frequency and operating a control of the rebroadcast of said carrier wave at the satellite station by the said amplified super-audible'frequency.

15. The method set forth in claim 13, in which the carrier wave is modulated with a plurality of super-audible frequencies which are demodulated at said satellite station, and selectively applying at said satellite station the respective'super-audible modulation components of the carrier wave there broadcast for the control of separate functions pertaining to the rebroadcast of such wave.

16. The method set forth in claim 13, which includes the modulation of the broadcast carrier wave with a plurality of super-audible frequencies, the demodulation, amplification and selection at the satellite station of such modulation frequencies as fall within a predetermined band, and effecting a first control of the broadcast from the satellite station in accordance with the pres 10 ence or absence of any frequency within such band, and effecting a second control of the broadcast from said satellite station according to the presence or absence of a particular frequency in such band.

17. The herein described method of covering without interference a shadowedarea in the service zone of a primary transmitter broadcasting a modulated carrier wave which comprises receiving the modulated carrier wave of the primary transmitter, generating an injection frequency, heterodyning the modulated carrier wave with said injection frequency, amplifying one of the resulting modulated beat frequencies, further heterodyning the amplified modulated beat frequency with the same identical injection frequency and amplifying and rebroadcasting to the said shadowed area that particular frequency resulting from such further heterodyning which is identical with the modulated carrier wave frequency originally heterodyned.

18. The method herein disclosed which comprises the broadcast reception of a modulated carrier wave of high frequency, generating an in- Jection frequency, heterodyning the received modulated carrier wave with the injection frequency, amplifying the lower of the resulting modulated beat frequencies, conducting the amplified modulated beat frequency to another point by wire, conducting by wire the identical said injection frequency to such other point, heterodyning the amplified modulated beat frequency at such other point with the injection frequency conducted to such other point, and amplifying and rebroadcasting from such other point the modulated beat frequency resulting from said second heterodyning step and which is identical with the frequency of the carrier wave originally received, the rebroadcasting from such other point being beamed to a shadowed area within the general zone of broadcast reception of the carrier wave first mentioned and screened from substantially all unshadowed portions of said PHILLIP B. LAESER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,881,483 Gillett Oct. 11, 1932 1,918,291 Schroter July 18, 1933 1,939,345 Gerth et a1. Dec. 12, 1933 1,994,288 Scheibell Mar. 12, 1935 2,028,857 Zworykin Jan. 28, 1936 2,106,806 Latimer et al Feb. 1, 1938 2,140,730 Batchelor Dec. 20, 1988 2,281,982 Leyn May 5, 1942 2,287,044 Kroger June 23, 1942 2,345,951 Smith Apr. 4, 1944 2,369,268 Trevor Feb. 13, 1945 2,401,333 Bumstead June 4, 1946 2,458,124 Wilmotte Jan. 4, 1949 FOREIGN PATENTS Number Country Date 373,919 Great Britain June 2, 1932 OTHER REFERENCES British Columbia's Broadcast Relay System, Electronics, September 1944, pages'92 to 97 and 304.

A Remote-Controlled Radio-Frequency Booster for a Broadcast Station, Proceedings of I. R. E, vol, 32, No. 9, September 1944, pages 525 to 533.

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US2835792 *Mar 20, 1951May 20, 1958Philips CorpTransmission system comprising a terminal station and a relay station
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Classifications
U.S. Classification455/22, 455/502
International ClassificationH04H20/67
Cooperative ClassificationH04H20/67
European ClassificationH04H20/67